In chemical plants, and the like, positioners are used as devices for controlling valve openings of regulator valves used in process control of flow rates (see Japanese Unexamined Patent Application Publication No. 2012-207756). A positioner is that which controls the valve opening of a regulator valve by calculating a deviation between a valve opening setpoint for a regulator valve, sent from a higher-level device, and a measured valve opening value for the regulator valve, to supply, to an operating device, a control signal that is based on this deviation, to cause the regulator valve to open or close.
Typically, a positioner has a structure wherein a dislocation detector, such as an angle sensor or a magnetic sensor, for detecting the actual opening of a regulator valve as an amount of dislocation of a valve rod of the regulator valve, a controller, made from a microcontroller, or the like, for performing signal processing based on an output signal, or the like, from the dislocation detector, an electropneumatic converting portion, for converting the electrical signal, produced by the controller, into a pneumatic signal, and a flow rate amplifying portion (a pilot relay) for amplifying and supplying, to the operating device, the pneumatic signal generated by the electropneumatic converting portion, are enclosed in a case made from metal (see Japanese Unexamined Patent Application Publication No. 2012-207756).